Probing the intergalactic magnetic field

Fast radio bursts (FRBs) are powerful flashes of astronomical radio waves that last just milliseconds and whose origins are a matter of debate. Ravi et al. discovered a FRB that is exceptionally bright (see the Perspective by Kaspi). This allowed them to measure how the polarization of the signal varies with wavelength (Faraday rotation). Combining this with the time delay of the flash at different wavelengths revealed the mean magnetic field along the line of sight. Assuming that the FRB originates from a colocated galaxy, the results constrain the intergalactic magnetic field and will inform models of galaxy formation and cosmology.

Abstract

Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5 ± 0.1 parsecs per cubic centimeter), exceptionally intense (120 ± 30 janskys), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 square arc minutes. On the basis of a low Faraday rotation (12.0 ± 0.7 radians per square meter), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nanogauss, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.